1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transmitters and/or receivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read and/or write terminal. Generally, transponders extract the power supply required by the electronic circuits included therein from a high-frequency field radiated by an antenna of the read-write terminal.
2. Discussion of the Related Art
FIG. 1 very schematically shows a conventional example of a data exchange system between a read/write terminal 1 and a transponder 10 of the type to which the present invention applies.
Generally, terminal 1 is essentially formed of a series oscillating circuit formed of an inductive resistor L1 in series with a capacitor C1 and a resistor R1. This series oscillating circuit is controlled by a device 2 including, among others and non-limitingly, an amplifier or antenna coupler, and a control circuit exploiting received data, especially provided with a modulator/demodulator and a microprocessor for processing the control signals and the data. Circuit 2 generally communicates with different input/output circuits (keyboard, screen, means of exchange with a server, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal generally draw the power necessary for their operation from a supply circuit (not shown) connected, for example, to the electric supply system.
A transponder 10, intended for cooperating with a terminal 1, essentially includes a parallel oscillating circuit formed of an inductive resistor L2, in parallel with a capacitor C2 between two input terminals 11, 12 of a control and processing circuit 13. Terminals 11, 12 are in practice connected to the input of a rectifying means (not shown), outputs of which form D.C. supply terminals of the circuits internal to the transponder. These circuits generally include, essentially, a microprocessor, a demodulator of the signals that may be received from terminal 1, and a modulator for transmitting information to the terminal.
The oscillating circuits of the terminal and of the transponder are generally tuned on the same frequency corresponding to the frequency of an excitation signal of the terminal's oscillating circuit. This high-frequency signal (for example, at 13.56 MHz) is not only used as a transmission carrier but also as a remote supply carrier for the transponder(s) located in the terminal's field. When a transponder 10 is located in the field of a terminal 1, a high-frequency voltage is generated across terminals 11 and 12 of the transponder's resonant circuit. This voltage, after being rectified and possibly clipped, is intended for providing the supply voltage of electronic circuits 13 of the transponder.
The high-frequency carrier transmitted by the terminal is generally modulated in amplitude by said terminal according to different coding techniques to transmit data and/or control signals to one or several transponders in the field. In return, the transmission of data from the transponder to a terminal is generally performed by modulating the load formed by resonant circuit L2, C2. This load variation is performed at the rate of a sub-carrier having a frequency (for example, 847.5 kHz) smaller than that of the carrier. This load variation can then be detected by the terminal in the form of an amplitude variation or of a phase variation by means, for example, of a measurement of the voltage across capacitor C1 or of the current in the oscillating circuit.
When idle, that is, when no transponder has entered its field, a terminal 1 periodically transmits a data message modulated on the high-frequency signal. This message is a request message intended for possible transponders. This request, or general call, belongs to a process needed for the initialization of a communication between a transponder and a terminal.
A difficulty in establishing a communication to one or several transponders is due to the fact that several electromagnetic transponders may be simultaneously located in the terminal's field.
Another difficulty is that transponders of different types or families (for example, transportation card, access card, electronic purse) use the same frequencies and, at least partly, the same communication protocols, which are normalized. Accordingly, a terminal must be able to determine, not only the number of transponders present in its field, but also the type of transponders, to select that or those with which it must communicate.
Such constraints require a loop operation of a control program of the terminal until all the transponders present in its field have been properly identified.
FIG. 2 schematically shows, in a simplified manner, a flowchart of initialization of one or several communications by a read-write terminal of the type to which the present invention applies.
After a starting, initialization and testing phase, a read-write terminal 1 of transponders starts (block 20, ST) a stand-by procedure during which it waits for the completion of a communication with at least one transponder. This procedure includes periodically sending (block 21) a request sequence (REQ) to the possible transponders present in the terminal's field. Upon each sending of an interrogation request 21, the reader monitors (block 22) the reception by its demodulator of an acknowledgement message (ATQ) coming from a transponder having entered its field. In the absence of any acknowledgement, the reader loops on the sending of a request 21. When its receives an acknowledgement ATQ, it switches to a mode of checking whether the transponder really is a transponder intended for it (block 23, TYPE), as well as to a possible anti-collision mode (block 24, ANTICOLLISION) to individualize several transponders that may be present in the field. Indeed, if several transponders are present in the terminal's field, they may respond at the same time or with a sufficiently short time interval to make the result of the demodulation by the reader unexploitable. Said reader must then either select a transponder with which it wishes to communicate, or assign different channels to the different transponders.
A communication only starts when the initialization and anti-collision process illustrated in FIG. 2 is over (block 26, E), that is, when the reader has detected (block 25, ALL) that it has identified all the transponders present in its field. Determining that a transponder belongs to a given family is part of this identification. As long as all the transponders have not been identified, interrogation requests are sent. If a transponder has been properly identified, it is placed in a state where it no longer acknowledges interrogation requests to avoid polluting the detection of the other possible transponders.
An initialization and anti-collision process such as briefly described in relation with FIG. 2 is well known. Illustrations of conventional methods are for example to be found in French patent applications no. 2,760,280 and 2,773,627 which are incorporated herein by reference.
The method illustrated in FIG. 2 is most often implemented by setting the maximum number of cards likely to be present in the transponder's field. As described, in particular, in French patent application No. 2,760,280, this number can be modified by the reader according to the results of exploitation of the anti-collision process (block 24) to increase the detection probabilities and to reduce the duration of the initialization process.
A disadvantage of conventional systems is that the determinations of the number and/of the type of transponders use a transmission of messages by the transponders. To determine the transponder's family, a software-implemented comparison of an identification transmitted by the transponder is used. In addition to the problems of program execution time, it is thus not checked whether the software identification is transmitted by a transponder or by a pirate device placed close to the terminal.